1. Field of the Invention
The present invention relates to a multiple beam light source device used in a multiple beam scanning optical apparatus for undergoing simultaneous writing with multiple beams, such as laser printers or digital copiers.
2. Related Background Art
FIG. 1 is a structural drawing of a conventional multiple beam writing apparatus used in laser printers or the like. A collimator lens 2, a cylindrical lens 3, and polygon mirror 4 for deflecting a plurality of laser beams are arranged in order in front of multiple beam semiconductor laser light source 1 having a plurality of emission points, and scanning lens 5 and photosensitive drum 6 serving as a recording medium are arranged in the area of reflection of polygon mirror 4. Synchronous detection mirror 7 for reflecting part of laser light deflectively reflected by the polygon mirror 4 is located between the scanning lens 5 and the photosensitive drum 6 and synchronous detection sensor 8 is placed in the area of reflection of the synchronous detection mirror 7. The output of laser driving circuit 9 is connected to the multiple beam semiconductor laser light source 1.
FIG. 2 is a perspective view of the conventional multiple beam semiconductor laser light source 1. Trapezoid stem 12 is provided on one surface of base stem 11, and semiconductor laser chip 13 and photodiode 14 are fixed on the trapezoid stem 12. Cap 15 is attached to the base stem 11 so as to cover the trapezoid stem 12 on which these semiconductor laser chip 13 and photodiode 14 are mounted, and window 16, which transmits laser beams La, Lb, is provided in a top surface of cap 15. Conduction terminals 17 for connecting the semiconductor laser chip 13 and photodiode 14 to a laser drive control system not illustrated are provided on the opposite surface of base stem 11. The photodiode 14 receives laser beams La', Lb' emitted backward from the semiconductor laser chip 13.
FIG. 3 is a front elevation of edge emitter type (end face emission type) semiconductor laser chip 13. One semiconductor laser chip 13 on the trapezoid stem 12 is made by vapor phase epitaxy, such as the MOCVD method or the MBE method, to form the n-type polarity of substrate 18, for example, from such a reason that less crystal defects are formed thereby in general. Specifically, the semiconductor laser chip 13 has the total height of about 150 .mu.m and is constructed in such structure that very thin crystal growth part 19, approximately 10 .mu.m, is deposited in lamination on the substrate 18 and active layer 20 is formed in the crystal growth part 19.
While a crystal is grown during fabrication of semiconductor laser chip 13, multiple emission points 21a, 21b are formed in the active layer 20. Metal electrodes are fixed to the top face of crystal growth part 19, and conduction wires 22a, 22b are soldered to the respective metal electrodes. As a consequence, the substrate 18 side becomes a common electrode in terms of electrical connection, while on the crystal growth part 19 side electrically separated electrodes are formed by effecting such crystal growth as to isolate the emission points 21a, 21b from each other or by providing a groove between them.
FIG. 4 is an explanatory drawing of beam spots P1, P2 made by the multiple laser beams emitted from the semiconductor light source 1 and scanning the area on the photosensitive drum 6. The beam spots P1, P2 are separated corresponding to adjacent scanning lines in the sub scanning direction and are also separated with a predetermined space S in the main scanning direction. Accordingly, the multiple emission points formed in the semiconductor laser chip 13 in the semiconductor laser light source 1 are positioned in such oblique relation as to be apart in the main scanning direction but as to be close to each other in the sub scanning direction.
When a laser drive signal is transmitted from the laser drive control system through the conduction terminals 17, an electric current is supplied through the conduction wires 22a, 22b to the semiconductor laser chip 13 to make the semiconductor laser chip 13 lase and the laser beams are emitted from the two emission points 21a, 21b disposed in the front face through the window 16.
In this way, the multiple laser beams emitted from the semiconductor laser light source 1 pass the collimator lens 2 and cylindrical lens 3 and thereafter are deflected by the polygon mirror 4 to scan. After that, they pass the scanning lens 5 to be focused on the photosensitive drum 6 and to scan the photosensitive drum 6. At this time, the laser beams emitted from the respective emission points 21a, 21b are positioned in an offset state in the sub scanning direction with respect to the writing scanning lines, so that the different, adjacent emission points are in charge of the respective, adjacent scanning line.
For synchronous detection in write scanning on the photosensitive drum 6, the synchronous detection sensor 8 captures the laser light emitted outside the effective write region and at a time immediately before write via the synchronous detection mirror 7. Therefore, the laser emission operation is carried out twice upon effective write and upon synchronous detection by drive of the laser drive circuit 9.
Further, APC (Automatic Power Control) is carried out for each emission point 21a, 21b in order to keep laser light amounts constant. This APC operation is carried out in such a way that the emission points 21a, 21b are made to lase separately and the beams La', Lb' emitted from the back surface of the semiconductor laser chip 13 at this time are detected by the photodiode 14 as changed in time division.
(1) However, the conventional example described above has such a drawback that, because the multiple emission points 21a, 21b are provided in one semiconductor laser chip 13, thermal crosstalk occurs from the adjacent emission point 21a, 21b and this results in variation of light amount between the emission points 21a, 21b.
(2) Since the electrodes of the emission points 21a, 21b of the semiconductor laser chip 13 are common on the cathode side or the n-type substrate 18 side, the conventional example has such a drawback that the number of choices is small for arranging the laser emission drive method in a laser emission circuit method adapted to image signals.
(3) Further, when a surface emission type semiconductor laser is used as the multiple beam semiconductor laser light source, directions of polarization of the respective emission points 21a, 21b are sometimes not identical. In such cases, the conventional example has such a drawback that nonuniformity appears in write light amounts on the occasion of scanning with their multiple laser beams through the optical elements including the polygon mirror 4 and scanning lens 5.